1. Field of the Invention
The present invention relates to a power source device that avoids causing vibration noise of a transformer under a light load, and an image forming apparatus including the power source device.
2. Description of the Related Art
Recently, energy saving products have been desired for devices. In particular, it has been desired to reduce standby power consumption, which is consumed in a state of connection to an outlet even without use in a normal operation. In a switching power source, typified by a flyback system, loss due to switching of a switching element accounts for most of the power consumption during waiting or under light load. As a scheme for solving this problem, a configuration is known that performs an intermittent operation where a switching interval under light load is longer than that in a continuous switching operation under normal load to reduce switching times per unit time, thereby reducing switching loss. According to Japanese Patent No. 4,415,052, further reduction in power consumption during waiting is realized by switching a resistor connected to an oscillator circuit frequency adjusting terminal of a power source IC in response to an external signal indicating that an apparatus enters a waiting mode to further increase the period of an intermittent switching operation.
However, the conventional technique has a possibility that the vibration frequency of a flyback transformer vibrating according to the intermittent period is low during an intermittent switching operation and is thus in a human audible range to thereby cause beat sound (vibration noise) of the transformer. In a light load state, the apparatus does not perform the normal operation and stays silent; the situation makes beat sound of the transformer further noticeable. Accordingly, measures against the beat sound in the light load state are required to be taken. To avoid the beat sound of the transformer with the conventional configuration, an operation with a vibration frequency exceeding the audible range can be considered. In this case, switching times increase. Accordingly, switching loss increases, in turn, increasing power consumption.
In Japanese Patent No. 4,415,052, the intermittent switching period is switched in response to an external signal indicating that a light load mode is entered. Accordingly, in an apparatus that has an individual difference in intermittent period due to components or substrates and has a plurality of power modes, it is difficult to take measures against beat sound of the transformer that occurs in the case where the vibration frequency of the transformer is low.